1. Field of the Invention
The present invention relates generally to a data rate controlling method in a mobile communication system, and in particular, to a reverse data rate controlling method.
2. Description of the Related Art
In general, IMT-2000 1xEV-DO (Evolution-Data Only) is a CDMA technique for providing high-speed data transmission only. Appropriate scheduling is required to efficiently transmit forward and reverse packet data in the 1xEV-DO system. Considering the air states and other environmental factors between a base station (BS) and mobile stations (MSs), the BS transmits data only to an MS at the best channel condition, to thereby maximize transmission throughput. For reverse packet data transmission, however, a plurality of MSs access the BS simultaneously. Therefore the BS must control overload within its capacity through appropriate control of reverse data flow and traffic congestion. 1xEV-DV (Evolution Data and Voice), a novel system under standardization, aiming at high-speed data transmission and voice service, must also control such overload.
In the 1xEV-DO system, an MS carries out reverse data transmission according to a RAB (Reverse Activity Bit) and a ReverseRateLimit (RRL) message received from a BS, and tells the BS its variable data rate via an RRI (Reverse Rate Indicator). The RRI indicates to the BS the data rate at which the reverse traffic data is being sent. The BS transmits time-division-multiplexed channels to the MS on an F-MAC (Forward Medium Access Control) channel: a pilot channel, an FAB (Forward Activity Bit) channel and a RAB channel. The RAB represents the congestion degree of the reverse link and a data rate available to the MS varies according to the RAB. The BS controls a data flow from the MS by commanding an increase/decrease in the reverse data rate using the RAB to thereby control the overload and capacity of the reverse link. The transmission time (or transmission period) of the RAB is determined byT mod RABlength  (1)where T is system time and RABlength is the length of the RAB expressed in the number of slots. Table 1 below lists binary values representing RAB lengths. The BS transmits one of the binary values to the MS in one slot and then the MS calculates a slot time when it receives the RAB on an F-MAC channel using the received RABlength information and the system time.
TABLE 1BinaryLength (slots)00 8011610321164
With the RAB received from the BS at the time calculated by Eq. (1), the MS determines a data rate available for the current reverse transmission. The MS receives PV (persistence vector) values in a message from the BS at or during a connection. The PV values are used in a PV test for increasing or decreasing a data rate when RAB=0 or RAB=1, respectively. When the PV test is passed, the MS doubles the current data rate or reduces it by half. When the PV test is failed, the MS maintains the current data rate. Specifically, when RAB=0 and the PV test is passed, the MS doubles the data rate. When RAB=1 and the PV test is passed, the MS reduces the data rate by half. The PV test is determined as passed if a random number satisfies a PV value.
From the system's perspective, this reverse data rate controlling method facilitates bandwidth and overload control. However, its uniform control for all MSs without considering their individual characteristics does not ensure efficient resources utilization.
The reverse data rate controlling method in the 1xEV-DO system will be described below. FIG. 1 is a flowchart illustrating the reverse data rate controlling method in an MS in the 1xEV-DO system.
The MS transmits initial data at a default data rate 9.6 kbps on the reverse link in step 10 and monitors an F-MAC channel in step 12. Upon receipt of a RAB on the F-MAC channel in step 14, the MS searches for an access probability Pi for the current data rate and generates a random number R in step 16. In step 18, the MS determines whether the RAB is 1. If the RAB is 1 and thus may result in a data rate decrease, the procedure goes to step 22, and if the RAB is 0 and thus may result in a data rate increase, it goes to step 20.
If the random number R is equal to or less than the access probability Pi, which implies that a PV test is passed, in step 20 or step 22, the MS increases or decrease its data rate by one level When step 20 goes to step 24, the MS increase data rate by one level, and when step 22 goes to step 26, the MS decrease the data rate by one level. Accordingly, a speed between levels has a double (or one-half) interval, which can be seen from Table. 2. In Table 2, it can be seen that the data rate is increased two times from 9.6 kbps through 153.6 kbps and is decreased by one-half from 153.6 kbps through 9.6 kbps. in step 24 or step 26. The MS transmits data at the changed data rate in step 28. If the changed data rate is lower than a data rate set in an RRL message, the MS transmits data on the set data rate 32 slots (53.33 ms) later. On the other hand, if the changed data rate is higher than the set data rate, the MS immediately changes its data rate to the set data rate.
After determining its data rate, the MS tells the BS the data rate in an RRI symbol as listed in Table 2 below. The data rate is one of 0, 9.6, 19.2, 38.4, 76.8 and 153.6 kbps.
TABLE 2Data rate (kbps)RRI symbol0 000 9.600119.201038.401176.8100153.6 101
To aid the MS in resetting its data rate, the BS transmits to the MS an RRL message having the structure shown in Table 3.
TABLE 3FieldLength (bits)Message ID829 occurrences of the following two fieldsRateLimitIncluded1RateLimit0 or 4ReservedVariable
Upon receipt of the RRL message, the MS resets its data rate by comparing the current data rate with a data rate set in the RRL message. 29 records may be inserted in the above RRL message and each record indicates a data rate assigned to a corresponding one of MACindexes 3 to 31. In Table 3, Message ID indicates the ID of the RRL message. RateLimitIncluded is a field indicating whether RateLimit is included in the RRL message. If RateLimit is included, RateLimitIncluded is set to 1, and otherwise, it is set to 0. RateLimit indicates a data rate assigned to a corresponding MS. The BS assigns RateLimit data rates listed below to MSs using four bits.
0 × 00kbps0 × 19.6kbps0 × 219.2kbps0 × 338.4kbps0 × 476.8kbps0 × 0153.6kbpsAll other values are invalid
During reverse data transmission, the MS monitors the F-MAC channel from the BS, especially the RAB on the F-MAC channel and resets its current data rate by performing a PV test.
FIG. 2 is a diagram illustrating data transmission/reception between an MS and 1xEV-DO sectors in its active set in the case of a sectored BS. In FIG. 2 “AT” refers to “Access Terminal” as used in the EV-DO standard, and corresponds to the BS. Referring to FIG. 2, F- and R-traffic channels and F- and R-MAC channels have been established between the MS and sector 1 with a connection opened between them. No F-traffic channels are assigned to the MS from sector 2 (up to six sectors 2 to 6) with no connection opened between them. In the 1xEV-DO system, the MS can maintain up to six BS sectors in its active set. Therefore, the MS monitors F-MAC channels from the active set sectors, especially RABs to determine its data rate.
Upon receipt of at least one RAB set to 1, the MS performs a PV test to determine whether to decrease its data rate. In the PV test, the MS generates a random number and compares it with a PV value for decreasing a data rate as defined by the BS at or during a connection. If the random number satisfies the PV value, the MS reduces its data rate by half, considering that the PV test is passed. On the contrary, if the PV test is failed, the MS maintains its data rate. If the data rate is lower than the default data rate, the MS sets its data rate to the default data rate. Meanwhile, if all the RABs are 0 and a PV test is passed, the data rate is doubled. If the PV test is failed, the MS maintains its data rate. If the increased data rate is higher than the highest available data rate, the MS sets its data rate to the highest data rate. In the case where the MS is limited in transmission power, it maintains its data rate. The RAB that leads to a one-half data rate increase or a half-data rate decrease on the reverse link is broadcast to MSs in time-division-multiplexing with an FAB on a forward common channel, the F-MAC channel. The MSs perform PV tests for increasing or decreasing their data rates uniformly according to the RAB.
In this reverse data rate controlling method for the 1xEV-DO system, reverse data rate is controlled based on probability since a PV test is performed according to a RAB. As a result, the full utilization of the reverse link is delayed. The uniform control without considering the individual statuses of MSs brings about resources inefficiency. The individual data rate control drastically increases overhead, degrading the system performance.